1. Field of the Invention
The present invention relates to a process for the production of a multipolar cable, in particular a multipolar cable for the transport or distribution of low or medium voltage electrical power.
Furthermore, the present invention relates to a multipolar cable, in particular a multipolar cable for the transport or distribution of low or medium voltage electrical power.
In the present description the term “low voltage” means a voltage below approximately 1 kV, the term “medium voltage” means a voltage between approximately 1 kV and approximately 30 kV, and the term “high voltage” means a voltage in excess of approximately 30 kV.
In the present description and in the claims which follow the term “core” of the cable means a semi-finite structure comprising a conductive element and at least one layer of electrical insulation placed in a position which is radially external to said conductive element. More particularly, when considering a cable for the transport or distribution of medium/high voltage electrical power, said “core” also comprises an internal semiconductive covering located in a position which is radially external to the conductive element, an external semiconductive covering located in a position which is radially external to the layer of electrical insulation and a metal screen in a position which is radially external to said external semiconductive covering.
Furthermore, for the purposes of the present description and the claims which follow, the term “multipolar cable” means a cable provided with at least a pair of “cores” as defined above. In greater detail, if the multipolar cable has a number of cores equal to two said cable is technically defined by using the term “bipolar cable”, and if the cores number three said cable is known as a “tripolar cable”, and so on.
The aforesaid cores, in a number which is predetermined according to the desired multipolar cable, once obtained are joined together to form a so-called “assembled element”
In the present description and in the claims which follow the term “assembled element” of a multipolar cable means a composite structure formed from the cores possessed by that cable. Preferably such a composite structure is obtained by helicoidally winding said cores together according to a predetermined pitch.
As a result of its nature, in that it is obtained by winding at least one pair of cores together, said assembled element has a plurality of interstitial zones which are defined by the spaces comprised between the cores. In other words, the winding of said cores gives rise to a plurality of voids, i.e. the interstitial zones, which, in a transverse cross section along the longitudinal length of the assembled element, define an external perimetral profile of the latter of noncircular type.
Therefore, in order to allow the correct application of the successive layers possessed by the multipolar cable, in a position which is radially external to said assembled element, the production process for a multipolar cable comprises the step of filling said interstitial zones so as to confer to the assembled element a regular transverse cross section, preferably of the circular type.
Said interstitial zones, which are also known with the term “star areas”, are generally filled with a filler of the conventional type, for example a polymeric material applied by extrusion.
Once said filling step has been completed and the semi-finished multipolar cable obtained so far is given a transverse cross section of the circular type, said cable is finished off with the application of at least a further layer, the nature of which, as well as the number of layers which can be used, depend on the type of multipolar cable to be obtained.
2. Description of the Related Art
For example, in accordance with a construction scheme of the conventional type performed by using techniques known in the art, in a radial position external to the aforesaid semi-finished cable obtained so far, it is possible to apply, in succession, a metal reinforcement (for example in the form of metal tapes or wires, generally made of steel, or in the form of a metal sheath, generally made of lead or aluminium) and an external polymeric sheath. In some cases the application of the metal reinforcement is preceded by the application of an inner polymeric sheath suitable to provide the assembled element with a mechanical protection from the metal reinforcement.
In accordance with a further form of embodiment, described in patent application WO 98/52197 in the name of the Applicant, a layer of expanded polymeric material of a suitable thickness can be applied in a position which is radially external to the aforesaid semi-finished cable obtained so far, said layer of expanded polymeric material being capable of conferring upon the aforesaid cable a high resistance to accidental impacts which might be suffered by the latter during the steps of cable transport or laying. In fact, said impacts can cause considerable damage to the cable structure (for example deformation of the insulating layer, detachment of the cable layers) determining, for example, changes in the electrical gradient of the insulating layer with a consequent reduction in its insulating capacity.
Furthermore, generally an external polymeric sheath suitable to confer to the cable a greater mechanical protection from the external environment is applied, according to known techniques, in a position which is radially external to said layer of expanded polymeric material.
Moreover, document WO 98/52197 cited above provides for the possibility of filling the aforesaid interstitial zones with an expanded polymeric material, similar to that which is used for a layer resistant to accidental impacts as illustrated above, instead of a conventional filler.
In fact, according to the Applicant, said embodiment presents some important advantages.
First of all, the use of an expanded polymeric material makes it possible to obtain a cable which is lighter than a similar cable whose interstitial zones are filled with a conventional filler.
Said aspect is very much more than negligible in that the possibility of providing for a cable which is lighter than a conventional one is reflected in greater ease of transport, and consequently in reduced transport costs, as well as in easier handling of the cable during the laying step. In this respect it is worthwhile emphasising that the less the overall weight of the cable to be installed (for example directly in a trench excavated into the ground or in a buried piping), the less will be the pulling force which is necessary to be applied to the cable in order to install it. Therefore, this means both lower installation costs and greater simplicity of the installation operations.
A second advantage is provided by the fact that the use of an expanded polymeric material in a radially internal position in the structure of the multipolar cable, i.e. in the aforesaid interstitial zones of the assembled element, as well as in a portion of the cable which is closer to its external surface, helps to confer to the cable itself a greater mechanical protection, i.e. a greater mechanical resistance to accidental impacts as defined above.
A further advantage lies in the fact that the use of an expanded polymeric material to replace a conventional filler helps to increase the flexibility of the cable, a fact which, once again, reflects in improved handling of the cable, with advantageous repercussions, as already mentioned, particularly during its installation.
However, the Applicant is of the opinion that applying an expandable polymeric material to the interstitial zones of the assembled element possessed by a multipolar cable is a complex operation which requires special care. In fact, an incorrect application of said material inside of the interstitial zones of the assembled element will result in the occurrence of unacceptable structural irregularities of the cable.
More particularly, the Applicant has found that the application of an expandable polymeric material and its expansion on a surface having an external perimetral profile of the non-circular type causes an irregular expansion of said material, and this tends to expand more in some zones than in others.
In other words, the polymeric material tends to expand more where there is more space available to do so.
For example, in the case a tripolar cable is considered, the assembled element, formed by the helicoidal winding of three separate cores, has three interstitial zones having cross sections of a substantially triangular shape, the base of each triangle being directed towards the extrados of the assembled element while the remaining two sides of said triangle are defined by the external profile of two adjacent cores of the cable. In this configuration, the polymeric material, which is designed to fill the interstitial zones and which is applied by extrusion in a position radially external to the assembled element, expands more in the portion of the interstitial zone which is closest to the base of the abovementioned triangle since in corrispondence with said base the polymeric material has the most space available to expand.
Non-uniform expansion of the polymeric material within each interstitial zone, i.e. at the extrados of the assembled element, confers to the semi-finished cable obtained hitherto, that is the assembled element plus the filler in the interstitial zones, a transverse cross section of irregular type, in that it has a plurality of protuberances, which in some cases are even very pronounced, where expansion of the polymeric material has been greatest.
For example, in the case of the tripolar cable mentioned above, the transverse cross section of the semi-finished cable obtained so far has an external perimetral profile which is substantially trilobate, the greater curvature of each lobe being located in correspondence of the triangle base defined above.
The occurrence of such a phenomenon is particularly undesirable as it brings about a plurality of disadvantages.
First of all, obtaining an assembled element which has an external perimetral profile of the non-circular type in a transverse cross section means that the layers which are subsequently applied in a position radially external to said assembled element, regardless of the nature of the multipolar cable to be produced, whether of the conventional type or as described in the abovementioned patent application WO 98/52197, will follow said non-circular external perimetral profile and thus result in the production of a finished cable having an irregular transverse cross section.
This result, apart from being unacceptable for the market from the purely aesthetic point of view, gives rise to a plurqlity of problems of a practical nature both during cable installation and storage. In the latter case many problems (larger dimensions, less loading capacity, instability of the turns wound on a spool) which might arise during a conventional operation of winding a cable of non-circular cross section can for example be envisaged.
A further disadvantage lies in the fact that a non-homogeneous distribution of the interstitial zones of the expanded polymeric material gives rise to the formation of zones which contain a greater concentration of material than other less well-endowed zones. Therefore, this means that said zones which are less well-endowed on the one hand show less mechanical resistance to impacts than the zones containing more material, and that, having a lesser thickness, they may be subjected to possible tearing during the extrusion step (more specifically at the exit from the extruder dies), said aspect causing the underlying assembled element to be exposed and consequently the formation of a non-circular transverse cross section of the cable obtained so far.
The Applicant has also found that, on the basis of the teaching provided by document WO 98/52197 mentioned above, whenever it is desired to proceed, with the same extrusion of expandable polymeric material, either to the filling the interstitial zones of the assembled element or the production of a layer resistant to accidental impacts, a non-uniform expansion of said material is obtained which inevitably causes the formation of a multipolar cable having a transverse cross section with an irregular external perimetral profile.